(1) DC Sputtering technique:

Figure 39.2 shows the arrangement used for direct current (DC) sputtering. In this case target and substrate oppose each other in the vacuum chamber having a distance of a few centimetres to few tens of centimeter. The target is connected to a negative output of a DC power supply, acting as the cathode whereas substrate and chamber walls act as anode. After the creation of argon atmosphere with a pressure of about 1 - 100 mTorr the gas discharge is ignited by applying a DC voltage. The created Ar⁺ ions are now accelerated toward the target and eject atoms from it. These atoms subsequently are deposited on the substrate. At low pressures, the mean free path between collisions is large, the ionization efficiency is low, and self-sustained discharges cannot be maintained below few mTorr. As the pressure increases at a fixed voltage, the electron mean free path is decreased, more ions are generated, and large current flow. If the pressure is too high, the sputtered atoms undergo increased collisional scattering resulting a low deposition process.

Figure 39.2: Schematic drawing of DC sputtering chamber.

The deposition rate is proportional to

• Power consumed

• Square of current density,

• 1/(electrode spacing).

DC sputtering works with all conductive target materials. However, it is not possible to sputter insulating materials with this technique because the positive charged ions can't flow through the insulator and hence the electric circuit is interrupted. So the potential at the cathode drops and the positive ions are no longer accelerated towards the target.

(2) RF Sputtering technique:

This sputtering process was invented as a mean of depositing insulating thin films. Quartz target was used to deposit SiO₂ film. The resistivity (ρ) of the quartz is 10¹⁶ Ω-cm. To draw a current density (J) of 0.1 mA/cm², cathode needs a voltage of V=0.1Jρ → V=10¹² Volts, which is physically impossible to produce and apply for practical works. This confirms that a target with resistivity exceeding 10⁶ Ω-cm cannot be sputtered using DC sources.